Process for producing zinc-chromium alloy-electroplated steel plate

ABSTRACT

A process for producing a Zn-Cr alloy-electroplated steel plate, comprising immersing a steel plate in an acidic bath containing Zn+2 ions and Cr+3 ions and subjecting the steel plate to electroplating in the bath, wherein 0.01 g/-20 g/l of an additive comprising one or more members selected from the group consisting of polyethyleneoxyphenol derivatives with a sulfonic group (-SO3H), sulfate group (-SO4H), amino group (-NH2), carboxyl group (-COOH), nitro group (-NO2) or halogen (-F, -Cl, -Br or -I) as a substituent in the benzene ring, alkyl sulfonic acid compounds and polyethyleneoxyalkyl sulfonic acid compounds are added to the bath. The plating process makes it possible to produce Zn-Cr alloy-properties steel plates at a low cost without the additives having any adverse effect on the properties of the plating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present intention relates to a process for producing acorrosion-resistant electroplated steel plate with excellent corrosionresistance for use in automobiles, electric appliances, buildingmaterials, etc. as well as excellent corrosion resistance after coating,and more specifically it relates to a process for producing a Zn--Cralloy-electroplated steel plate.

2. Description of the Related Art

Methods for producing electroplated steel plates wherein chromium isincluded in zinc or zinc-based alloy platings are disclosed for examplein Japanese Unexamined Patent Publication (Kokai) Nos. 63-243295,1-191797 and 3-120393, according to which an organic additive is used inthe plating bath to codeposit the chromium. The action mechanism of theadditive is not fully understood, but the following two theories havebeen set forth.

A. The additive exhibits its codepositing effect by adsorbing onto theelectrode surface.

B. The additive forms a complex with Cr⁺³ ions, and the complex exhibitsa codepositing effect.

In the case of theory A, the additive may be incorporated into theplated film during electrodeposition of the alloy, causing changes inthe properties of the plating and deteriorating the adhesion of theplating. The concentration of the additive is consequently lowered, andit may be impossible to prepare an alloy with a consistent composition.In the case of theory B, a large amount of the additive is necessary toform the complex with the Cr⁺³ ions, which is not suitable foreconomical production. Furthermore, a long time is required to form thecomplex when the Cr⁺³ ion is supplied during continuous operation, andcontrol of the plating bath is difficult.

No measures have yet been taken to solve the aforementioned problemsinvolved with adding the additive to the plating bath.

As mentioned above, (a) the additive in the plating bath must contributeto the alloy electrodeposition without being incorporated into theplating during the electrodeposition, and must be returned to the bulkfrom the vicinity of the steel plate surface immediately after the alloyelectrodeposition, (b) there must be a mechanism to prevent variation inthe additive concentration before and after the electrodeposition, (c)the additive must be effective in small amounts, and (d) the control ofthe additive must be easy, requiring no pretreatment such as priorelectrolysis, aging, etc. of the plating bath.

SUMMARY OF THE INVENTION

In light of these problems, the present invention provides a process forproducing Zn--Cr alloy-electroplated steel plates using an additivewhich does not influence the properties of the plating, which iseconomical and advantageous in operation, which may be used immediatelyafter preparation of the plating bath, and which may be easilycontrolled.

The present invention provides a process of producing a Zn--Cralloy-electroplated steel plate, comprising immersing a steel plate inan acidic bath containing Zn⁺² ions and Cr⁺³ ions and subjecting thesteel plate to electroplating in the bath, wherein 0.01 g/l-20 g/l of anadditive comprising one or more members selected from the groupconsisting of polyethyleneoxyphenol derivatives with a sulfonic group(--SO₃ H), sulfate group (--SO₄ H), amino group (--NH₂), carboxyl group(--COOH), nitro group (--NO₂) or halogen (--F, --Cl, --Br or --I) as asubstituent in the benzene ring, alkyl sulfonic acid compounds andpolyethyleneoxyalkyl sulfonic acid compounds are added to the bath.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, the polyethyleneoxyphenolderivative, alkyl sulfonic acid compound and polyethyleneoxyalkylsulfonic acid compound added to the plating bath as additives may havethe following structures, respectively.

    ______________________________________                                         ##STR1##       Polyethyleneoxyphenol derivative m = 2-100                    R = SO.sub.3 H, SO.sub.4 H, NH.sub.2, COOH, NO.sub.2, F, Cl,                  Br or I                                                                       CH.sub.3 (CH.sub.2).sub.n SO.sub.3 H                                                          Alkyl sulfonic acid compound                                                  n = 5-19                                                      CH.sub.3 R.sub.1 R.sub.2 CH.sub.2 SO.sub.3 H                                                  Polyethyleneoxyalkyl sulfonic                                                 acid compound                                                 R.sub.1 = (CH.sub.2).sub.a                                                                    a = 5-19                                                      R.sub.2 = (CH.sub.2 CH.sub.2 O).sub.b                                                         b = 2-100                                                     or                                                                            R.sub.1 = (CH.sub.2 CH.sub.2 O).sub.c                                                         c = 2-100                                                     R.sub.2 = (CH.sub.2).sub.d                                                                    d = 5-19                                                      ______________________________________                                    

These additives usable for the present invention are believed to formmolecular aggregates known as micelles in aqueous solution. Thehydrophilic groups are oriented toward the aqueous solution. The detailsof the structure of the molecular aggregates of the above-mentionedcompounds are not fully understood, but the hydrophilic portion has asite which attaches to Cr³⁺ ions. The force of attraction here differsfrom the coordinate bonding for forming the complex in that it is a weakelectrostatic bond, and therefore electrolysis of the Cr³⁺ ions occursmore readily, giving a higher current efficiency for the plating thanhitherto achieved. The mechanism of electrodeposition of Zn and Cr isnot yet fully understood, but is assumed to be as follows. That is, whenthe micelles with the attached Cr³⁺ ions approach the surface of thesteel plate, the Cr³⁺ ions are deposited and the micelles, free from themetal ions, migrate back to the bulk. In the bulk, Cr³⁺ ions are againattached and the same process is repeated. Meanwhile, Zn²⁺ ions, whichtend naturally to be electrodeposited, are deposited without theinteraction with the additive. The deposited Cr and Zn are alloyed,becoming a Zn--Cr alloy.

The additive is not incorporated into the plated film with the alloyduring electrodeposition. This is because, first of all, the additivedoes not have the mechanism of the alloy electrodeposition by which toadhere directly to the steel plate, and thus the micelles with theattached Cr³⁺ ions merely deposit them when they approach the vicinityof the steel plate surface. Also, since the size of the micelles isusually very large compared with the zinc and chromium atoms, it isinconceivable for the molecular aggregates to be incorporated into theplated film. Incidentally, the size of the micelles is not specifiedsince it varies depending on the type, concentration, etc. of theadditive.

The result is that the additive undergoes no reduction in concentrationby incorporation into the film, and therefore the plating compositionremains constant during continuous production and the propterties of theplating do not vary.

According to the present invention the polyethyleneoxyphenol derivativeswith a substituent in the benzene ring are limited to those whosesubstituents are a sulfonic group (--SO₃ H), sulfate group (--SO₄ H),amino group (--NH₂), carboxyl group (--COOH), nitro group (--NO₂) orhalogen (--F, --Cl, --Br or --I), for the following reason.

If the substituent is strongly hydrophobic, such as an alkyl group,micelles will not be formed and a suspension may result. This poses therisk of irregularities in the plating. Thus it is necessary that thesubstituent have a certain degree of hydrophilicity, and as a result ofdiligent research we have found that sulfonic groups (--SO₃ H), sulfategroups (--SO₄ H), amino groups (--NH₂), carboxyl groups (--COOH), nitrogroups (--NO2) and halogens (--F, --Cl, --Br and --I) are preferable.

The substituent may form any type of salt so long as it does notincrease the hydrophobicity as in the case of a sulfuric acid ester(--SO₄ R, R=alkyl group). Examples to be used include sodium sulfonate(--SO₃ Na), ammonium sulfonate (--SO₃ NH₄), sodium sulfate (--SO₃ Na),ammonium sulfate (--SO₄ NH₄), amine hydrochloride (--NH₃ Cl), sodiumcarboxylate (--COONa) and ammonium carboxylate (--COONH₄).

The additives mentioned above naturally exhibit their effect when usedalone, and also 2 or more thereof may be used in admixture withoutimpairing their mutual effects.

According to the present invention, control of the plating bath isrelatively straight forward. In conventional Zn--Cr alloy electroplatingbaths, the structures of the Cr³⁺ complex include complexes with theadditive, hydrated complexes and hydrated complexes including someanions, and since the hydrated complexes are in the form of mononuclear,binuclear and polynuclear complexes, the proportions vary depending onthe standing temperature of the bath and the time. As a result, thecomposition of the plate obtained when the proportion of complexes inthe bath is not constant has caused the problem of instability. However,since the present invention has a mechanism for electrodepositing Cr bymaking use of the phenomenon of Cr³⁺ ions attaching to micelles, it ispossible to prepare a consistently constant alloy with the samecomposition regardless of the state of the Cr³⁺ ion.

The minimum required amount of the additive for formation of themicelles is a very small amount of 0.01 g/l, which is also suitable interms of economical operation. Excessive addition leads to an increasein the viscosity of the solution and lower electrical conductivity, andin some cases there is a risk of a suspension resulting withoutdissolution of the additive, and therefore the amount is preferably nomore than 20 g/l.

According to the present invention, a small amount (0.1 mol/l or less)of a metal ion such as Ni²⁺, Co²⁺, Fe²⁺, Mn²⁺ or Pb²⁺ may be added tothe plating bath if necessary for electrodeposition of a third componentin the plated film.

The composition of the Zn--Cr alloy plating obtained according to thepresent invention may be varied by changing the composition of theplating bath and the current density. However, from the standpoint ofcorrosion resistance the Cr content is preferably 5 wt % or more, and itis preferably no more than 40 wt % since if the content exceeds this theplating becomes too powdery during working, leading to poor powderingproperties. The plating bath may be either a sulfuric acid bath or ahydrochloric acid bath. The steel base to be plated is not specificallyrestricted, and there are no conditions on the method of producing thesteel components or steel plate.

The present invention will now be explained by way of the followingexamples.

For Zn--Cr alloy-plating of a cold stretched steel plate with a platethickness of 0.8 mm, a plating bath containing 0.6 mol/l of Zn²⁺ ion and0.4 mol/l of Cr³⁺ ion with an acidity of pH 1.5 was used, with theplating being conducted at a bath temperature of 50° C. and a currentdensity of 100 A/dm². The additives listed in Tables 1-6 were added tothe plating bath at this time to obtain various Zn--Cr alloy-platedsteel plates. The properties of the resultant plated steel plates wereevaluated in terms of workability and corrosion resistance. Sincecodeposition of the additives in the Zn--Cr alloy-plated layer causingcarbon contamination results in deterioration, the workability wasdetermined as a measure of the presence or absence of additivecodeposition. The evaluation was made based on the peeling of theplating at the bent sections upon adherence bending. The 3 levels ofevaluation were ⊚ (no peeling), ∘ (slight peeling) and ▴ (peeling). Theresults are shown in Tables 1-6. Also, the corrosion resistance of theplating was determined by measuring the proportion (%) of red rustproduced during 1008 hours, using the salt spray test according toJIS2371, with a production of red rust of up to 20% evaluated as ∘ andover 20% evaluated as x.

The ease of control of the plating bath was evaluated in terms of thestability of the resulting plating composition with time. That is, thedifference between the plating composition obtained upon platingimmediately after preparation of the plating bath and the platingcomposition obtained upon plating after aging at 50° C. for 48 hours wasevaluated, with a difference of up to 1% denoted by ∘, and over 1%denoted as x.

                                      TABLE 1                                     __________________________________________________________________________                               Wt % of                                                                       chromium                                                                            Corro-                                                                  in    sion                                                                    plated                                                                              resis-                                                                            Worka-                                                                             Stability                           Exs. No.                                                                           Additive (g/l) Substituent                                                                          film  tance                                                                             bility                                                                             with time                           __________________________________________________________________________    1    Polyethyleneoxyphenol                                                                     0.01                                                                             Sulfonate                                                                            8     ∘                                                                     ⊚                                                                   ∘                            derivative     group                                                     2    Polyethyleneoxyphenol                                                                     0.10      10    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               3    Polyethyleneoxyphenol                                                                     0.50      14    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               4    Polyethyleneoxyphenol                                                                     1.00      17    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               5    Polyethyleneoxyphenol                                                                     5.00      24    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               6    Polyethyleneoxyphenol                                                                     10.00     30    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               7    Polyethyleneoxyphenol                                                                     20.00     34    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               8    Polyethyleneoxyphenol                                                                     0.01                                                                             Sulfate                                                                              8     ∘                                                                     ⊚                                                                   ∘                            derivative     group                                                     9    Polyethyleneoxyphenol                                                                     0.10      11    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               10   Polyethyleneoxyphenol                                                                     0.50      15    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               11   Polyethyleneoxyphenol                                                                     1.00      19    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               12   Polyethyleneoxyphenol                                                                     10.00     29    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               13   Polyethyleneoxyphenol                                                                     20.00     32    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               14   Polyethyleneoxyphenol                                                                     0.01                                                                             Amino group                                                                          7     ∘                                                                     ⊚                                                                   ∘                            derivative                                                               15   Polyethyleneoxyphenol                                                                     0.10      10    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               16   Polyethyleneoxyphenol                                                                     0.50      13    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               17   Polyethyleneoxyphenol                                                                     1.00      18    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               18   Polyethyleneoxyphenol                                                                     10.00     27    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               19   Polyethyleneoxyphenol                                                                     20.00     30    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               20   Polyethyleneoxyphenol                                                                     0.01                                                                             Carboxyl                                                                             7     ∘                                                                     ⊚                                                                   ∘                            derivative     group                                                     21   Polyethyleneoxyphenol                                                                     0.10      9     ∘                                                                     ⊚                                                                   ∘                            derivative                                                               22   Polyethyleneoxyphenol                                                                     0.50      12    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               23   Polyethyleneoxyphenol                                                                     1.00      15    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               24   Polyethyleneoxyphenol                                                                     10.00     21    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               25   Polyethyleneoxyphenol                                                                     20.00     25    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               26   Polyethyleneoxyphenol                                                                     0.01                                                                             Nitro group                                                                          8     ∘                                                                     ⊚                                                                   ∘                            derivative                                                               27   Polyethyleneoxyphenol                                                                     0.10      11    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               28   Polyethyleneoxyphenol                                                                     0.50      15    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               29   Polyethyleneoxyphenol                                                                     1.00      19    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               30   Polyethyleneoxyphenol                                                                     10.00     27    ∘                                                                     ⊚                                                                   ∘                            derivative                                                               __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                              Wt % of                                                                       chromium                                                                            Corro-                                                                  in    sion                                                                    plated                                                                              resis-                                                                            Worka-                                                                             Stability                            Exs. No.                                                                           Additive g/l   Substituent                                                                         film  tance                                                                             bility                                                                             with time                            __________________________________________________________________________    31   Polyethyleneoxyphenol                                                                     20.00                                                                            Nitro 31    ∘                                                                     ⊚                                                                   ∘                             derivative     group                                                     32   Polyethyleneoxyphenol                                                                     0.01                                                                             --Cl  6     ∘                                                                     ⊚                                                                   ∘                             derivative                                                               33   Polyethyleneoxyphenol                                                                     0.10     8     ∘                                                                     ⊚                                                                   ∘                             derivative                                                               34   Polyethyleneoxyphenol                                                                     0.50     11    ∘                                                                     ⊚                                                                   ∘                             derivative                                                               35   Polyethyleneoxyphenol                                                                     1.00     14    ∘                                                                     ⊚                                                                   ∘                             derivative                                                               36   Polyethyleneoxyphenol                                                                     10.00    23    ∘                                                                     ⊚                                                                   ∘                             derivative                                                               37   Polyethyleneoxyphenol                                                                     20.00                                                                            --Br  27    ∘                                                                     ⊚                                                                   ∘                             derivative                                                               38   Alkyl sulfonic acid                                                                       0.01     6     ∘                                                                     ⊚                                                                   ∘                        39   Alkyl sulfonic acid                                                                       0.10     8     ∘                                                                     ⊚                                                                   ∘                        40   Alkyl sulfonic acid                                                                       0.50     9     ∘                                                                     ⊚                                                                   ∘                        41   Alkyl sulfonic acid                                                                       1.00     12    ∘                                                                     ⊚                                                                   ∘                        42   Alkyl sulfonic acid                                                                       10.00    16    ∘                                                                     ⊚                                                                   ∘                        43   Alkyl sulfonic acid                                                                       20.00    21    ∘                                                                     ⊚                                                                   ∘                        44   Polyethyleneoxyalkyl                                                                      0.01     11    ∘                                                                     ⊚                                                                   ∘                             sulfonic acid                                                            45   Polyethyleneoxyalkyl                                                                      0.10     16    ∘                                                                     ⊚                                                                   ∘                             sulfonic acid                                                            46   Polyethyleneoxyalkyl                                                                      0.50     20    ∘                                                                     ⊚                                                                   ∘                             sulfonic acid                                                            47   Polyethyleneoxyalkyl                                                                      1.00     27    ∘                                                                     ⊚                                                                   ∘                             sulfonic acid                                                            48   Polyethyleneoxyalkyl                                                                      10.00    31    ∘                                                                     ⊚                                                                   ∘                             sulfonic acid                                                            49   Polyethyleneoxyalkyl                                                                      20.00    34    ∘                                                                     ⊚                                                                   ∘                             sulfonic acid                                                            50   Polyethyleneoxyphenol                                                                     0.01                                                                             Sulfonate                                                                           9     ∘                                                                     ⊚                                                                   ∘                             derivative     group                                                          Alkyl sulfonic acid                                                                       0.01                                                         51   Polyethyleneoxyphenol                                                                     0.01     9     ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            52   Polyethyleneoxyphenol                                                                     0.10     12    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic acid                                                                       0.01                                                         53   Polyethyleneoxyphenol                                                                     0.10     11    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            54   Polyethyleneoxyphenol                                                                     0.10     11    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic acid                                                                       1.00                                                         55   Polyethyleneoxyphenol                                                                     0.10     22    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      1.00                                                              sulfonic acid                                                            __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                              Wt % of                                                                       chromium                                                                            Corro-                                                                  in    sion                                                                    plated                                                                              resis-                                                                            Worka-                                                                             Stability                            Exs. No.                                                                           Additive (g/l) Substituent                                                                         film  tance                                                                             bility                                                                             with time                            __________________________________________________________________________    56   Polyethyleneoxyphenol                                                                     0.50                                                                             Sulfonate                                                                           12    ∘                                                                     ⊚                                                                   ∘                             derivative     group                                                          Alkyl sulfonic acid                                                                       0.01                                                         57   Polyethyleneoxyphenol                                                                     0.50     16    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            58   Polyethyleneoxyphenol                                                                     1.00     17    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic acid                                                                       0.01                                                         59   Polyethyleneoxyphenol                                                                     1.00     19    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            60   Polyethyleneoxyphenol                                                                     10.00    28    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic acid                                                                       0.01                                                         61   Polyethyleneoxyphenol                                                                     10.00    32    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            62   Polyethyleneoxyphenol                                                                     0.10                                                                             Sulfate                                                                             10    ∘                                                                     ⊚                                                                   ∘                             derivative     group                                                          Alkyl sulfonic acid                                                                       0.01                                                         63   Polyethyleneoxyphenol                                                                     0.10     12    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            64   Polyethyleneoxyphenol                                                                     1.00     18    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic acid                                                                       0.01                                                         65   Polyethyleneoxyphenol                                                                     1.00     20    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            66   Polyethyleneoxyphenol                                                                     10.00    27    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic acid                                                                       0.01                                                         67   Polyethyleneoxyphenol                                                                     10.00    33    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            68   Polyethyleneoxyphenol                                                                     0.01                                                                             Amino 7     ∘                                                                     ⊚                                                                   ∘                             derivative     group                                                          Alkyl sulfonic acid                                                                       0.01                                                         69   Polyethyleneoxyphenol                                                                     0.01     9     ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      1.01                                                              sulfonic acid                                                            70   Polyethyleneoxyphenol                                                                     0.50                                                                             Nitro 14    ∘                                                                     ⊚                                                                   ∘                             derivative     group                                                          Alkyl sulfonic acid                                                                       0.01                                                         __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                              Wt % of                                                                       chromium                                                                            Corro-                                                                  in    sion                                                                    plated                                                                              resis-                                                                            Worka-                                                                             Stability                            Exs. No.                                                                           Additive (g/l) Substituent                                                                         film  tance                                                                             bility                                                                             with time                            __________________________________________________________________________    71   Polyethyleneoxyphenol                                                                     0.50                                                                             Nitro 15    ∘                                                                     ⊚                                                                   ∘                             derivative     group                                                          Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            72   Polyethyleneoxyphenol                                                                     1.00     18    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic                                                                            0.01                                                              acid                                                                     73   Polyethyleneoxyphenol                                                                     1.00     21    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            74   Polyethyleneoxyphenol                                                                     10.00    25    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic                                                                            0.01                                                              acid                                                                     75   Polyethyleneoxyphenol                                                                     10.00    29    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            76   Polyethyleneoxyphenol                                                                     0.01                                                                             Carboxyl                                                                            8     ∘                                                                     ⊚                                                                   ∘                             derivative     group                                                          Alkyl sulfonic                                                                            0.01                                                              acid                                                                     77   Polyethyleneoxyphenol                                                                     0.01     8     ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            78   Polyethyleneoxyphenol                                                                     0.50     13    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic                                                                            0.01                                                              acid                                                                     79   Polyethyleneoxyphenol                                                                     0.50     16    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            80   Polyethyleneoxyphenol                                                                     1.00     17    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic                                                                            0.01                                                              acid                                                                     81   Polyethyleneoxyphenol                                                                     1.00     19    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            82   Polyethyleneoxyphenol                                                                     10.00    22    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Alkyl sulfonic                                                                            0.01                                                              acid                                                                     83   Polyethyleneoxyphenol                                                                     10.00    25    ∘                                                                     ⊚                                                                   ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.01                                                              sulfonic acid                                                            __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________                                 Wt % of                                                                       chromium                                                                            Corro-                                                                  in    sion                                                                    plated                                                                              resis-                                                                            Worka-                                                                             Stability                         Comp. Exs. No.                                                                        Additive (g/l) Substituent                                                                         film  tance                                                                             bility                                                                             with time                         __________________________________________________________________________    1       Polyethyleneoxyphenol                                                                     0.001                                                                            Sulfo-                                                                              2     x   ⊚                                                                   ∘                             derivative     nate                                                                          group                                                  2       Polyethyleneoxyphenol                                                                     0.001                                                                            Sulfate                                                                             2     x   ⊚                                                                   ∘                             derivative     group                                                  3       Polyethyleneoxyphenol                                                                     0.001                                                                            Amino                                                          derivative     group 1     x   ∘                                                                      ∘                     4       Polyethyleneoxyphenol                                                                     0.001                                                                            Carboxyl                                                                            0     x   ∘                                                                      ∘                             derivative     group                                                  5       Polyethyleneoxyphenol                                                                     0.001                                                                            Nitro 1     x   ⊚                                                                   ∘                             derivative     group                                                  6       Polyethyleneoxyphenol                                                                     0.001                                                                            --Cl  0     x   ∘                                                                      ∘                             derivative                                                            7       Alkyl sulfonic acid                                                                       0.001    0     x   ∘                                                                      ∘                     8       Polyethyleneoxyalkyl                                                                      0.001    2     x   ∘                                                                      ∘                             sulfonic acid                                                         9       Polyethyleneoxyphenol                                                                     0.001                                                                            Sulfate                                                                             1     x   ∘                                                                      ∘                             derivative     group                                                          Alkyl sulfonic acid                                                                       0.001                                                     10      Polyethyleneoxyphenol                                                                     0.001                                                                            Amino 0     x   ∘                                                                      ∘                             derivative     group                                                          Alkyl sulfonic acid                                                                       0.001                                                     11      Polyethyleneoxyphenol                                                                     0.001                                                                            Carboxyl                                                                            0     x   ∘                                                                      ∘                             derivative     group                                                          Alkyl sulfonic acid                                                                       0.001                                                     12      Polyethyleneoxyphenol                                                                     0.001                                                                            Nitro 1     x   ∘                                                                      ∘                             derivative     group                                                          Alkyl sulfonic acid                                                                       0.001                                                     13      Polyethyleneoxyphenol                                                                     0.001                                                                            --Cl  0     x   ∘                                                                      ∘                             derivative                                                                    Alkyl sulfonic acid                                                                       0.001                                                     14      Polyethyleneoxyphenol                                                                     0.001                                                                            Sulfo-                                                                              0     x   ∘                                                                      ∘                             derivative     nate                                                           Polyethyleneoxyalkyl                                                                      0.001                                                                            group                                                          sulfonic acid                                                         __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________                                 Wt % of                                                                       chromium                                                                            Corro-                                                                  in    sion                                                                    plated                                                                              resis-                                                                            Worka-                                                                             Stability                         Comp. Exs. No.                                                                        Additive (g/l) Substituent                                                                         film  tance                                                                             bility                                                                             with time                         __________________________________________________________________________    15      Polyethyleneoxyphenol                                                                     0.001                                                                            Sulfate                                                                             1     x   ∘                                                                      ∘                             derivative     group                                                          Polyethyleneoxyalkyl                                                                      0.001                                                             sulfonic acid                                                         16      Polyethyleneoxyphenol                                                                     0.001                                                                            Amino 1     x   ∘                                                                      ∘                             derivative     group                                                          Polyethyleneoxyalkyl                                                                      0.001                                                             sulfonic acid                                                         17      Polyethyleneoxyphenol                                                                     0.001                                                                            Carboxyl                                                                            0     x   ∘                                                                      ∘                             derivative     group                                                          Polyethyleneoxyalkyl                                                                      0.001                                                             sulfonic acid                                                         18      Polyethyleneoxyphenol                                                                     0.001                                                                            Nitro 0     x   ∘                                                                      ∘                             derivative     group                                                          Polyethyleneoxyalkyl                                                                      0.001                                                             sulfonic acid                                                         19      Polyethyleneoxyphenol                                                                     0.001                                                                            --Br  0     x   ∘                                                                      ∘                             derivative                                                                    Polyethyleneoxyalkyl                                                                      0.001                                                             sulfonic acid                                                         20      Polyethyleneoxyalkyl                                                                      0.001    0     x   ∘                                                                      ∘                             sulfonic acid                                                                 Alkyl sulfonic                                                                            0.001                                                             acid                                                                  21      Polyethyleneoxyphenol                                                                     0.001                                                                            Sulfo-                                                                              0     x   ∘                                                                      ∘                             derivative     nate                                                           Polyethyleneoxyalkyl                                                                      0.001                                                                            group                                                          sulfonic acid                                                                 Alkyl sulfonic                                                                            0.001                                                             acid                                                                  22      Tartaric acid                                                                             4        0.01  x   ∘                                                                      x                                 23      Tartaric acid                                                                             10       7     ∘                                                                     x    x                                 24      Malic acid  4        0.1   x   ∘                                                                      x                                 25      Malic acid  9        5     ∘                                                                     x    x                                 __________________________________________________________________________

In Examples 1-83, specific Zn--Cr alloys with a Cr wt % of the platedfilm of up to 6%-34% were obtained. Also, the corrosion resistances andplating adhesions were excellent. The stability of the platingcompositions with time were also excellent.

On the other hand, as shown by the Comparative Examples 1-21, with onlytrace amounts of the additives, almost no chromium was deposited in theplating films, and therefore these do exhibit satisfactory corrosionresistance; however, as shown by the Comparative Examples 22-25, whenonly small amounts of the additives that form complexes with Cr³⁺ ionsare added, chromium is not deposited in the plating layer and thus thecorrosion resistance is poor, whereas with larger amounts of theadditive, chromium is deposited with improvement in the corrosionresistance but the workability is impaired. The stability of the platingcomposition with time is also poor.

As is clear from the above explanation, in the process of the presentinvention, the additive has no adverse effect whatsoever on theproperties of the plating, and therefore Zn--Cr alloy-electroplating maybe performed at a low cost; the process is therefore suitable for theproduction of Zn--Cr alloy-electroplatings.

We claim:
 1. A process for producing a Zn--Cr alloy-electroplated steelplate, comprising immersing a steel plate in an acidic bath containingZn⁺² ions and Cr⁺³ ions and subjecting the steel plate to electroplatingin the bath to from a Zn--Cr alloy-electroplated steel plate, wherein0.01 g/l-20 g/l of an additive comprising a polyethyleneoxyphenolderivative with at least one of a sulfonic group (--SO₃ H), sulfategroup (--SO₄ H), amino group (--NH₂), carboxyl group (--COOH), nitrogroup (--NO₂) or halogen (--F, --Cl, --Br or --I) as a substituent inthe benzene ring is added to the bath to accelerate the Cr Codepositionand to improve the stability of the bath.
 2. A process according toclaim 1, wherein the polyethyleneoxyphenol derivative is selected fromthe compounds represented by the following formula ##STR2## wherein R is--SO₃ H, --SO₄ H, NH₂, --COOH, --NO₂, --F, --Cl, --Br, or --I, and m isan integer of 1 to
 100. 3. A process according to claim 1, wherein theZn--Cr alloy of the electroplated steel plate has a Cr content of 5 to40% by weight.